Such a method is known. In this method it is determined by means of the ground-fault detector if a ground fault is present in the system. Such a ground-fault detector or supervising means is connected to the induction coil and includes, for instance, a signal source and a comparator that measures the resistance to ground of the induction coil and its power supply circuit. The means has a window comparator with an adjustable upper and lower limit that compares the measured resistance against upper and lower thresholds. If the measured resistance drops below the lower limit, the ground-fault detector provides an alarm that shuts the power down and signals to the operator that there is a ground fault. If the measured resistance increases above the upper limit, the ground-fault detector provides an alarm that shuts the power down and signals to the operator that the ground resistance measurement circuit is defective.
Such a ground fault can be generated by a connection between the coil and ground, between the power supply and ground or between other electrically live components located outside the coil and ground. Furthermore, a ground fault can belong to the induction furnace coil or power supply or other electrically live components located outside the coil. Moreover, a coil ground fault can be caused by penetration of the molten metal through the refractory lining. In this case, a path to ground is provided by the at least one bath ground electrode.
A problem with the prior art of such a ground-fault detector or controllers of induction furnaces is that the occurrence of a ground fault is only detected and indicated, however, it is not indicated where the ground fault occurs. In the case of a ground fault the operator has to examine corresponding zones in the range of the furnace for the search of defects. However, this is time consuming and expensive.